High density optical recording is becoming more and more prevalent and such systems typically employ a light beam which is focused on a recording medium to produce a pit or a mark. It is of course important that the light beam be "in focus" when recording takes place so that the resulting marks are of high quality. Heretofore, this has been accomplished in the following manner. The light beam from an optical source, such as a laser diode, is reflected by the recording medium and in turn illuminates two or more photodetectors. Differences in focus position produce variations in the illumination pattern on the photodetectors. As a result of differences in illumination, these photodetectors produce different signals, from which a focus error signal is computed. The write focus "offset" is defined as the focus error signal which occurs when the recording beam is in focus. If the focus error signal during data recording is different from the predetermined offset value, then there is an error and the position of the optical head is adjusted. For good quality recording in high data density systems, such as recordable CDs, this focus offset value must be determined accurately. The focus offset is typically determined empirically for each optical head to compensate for optical alignment inaccuracies. One way this has been accomplished is to record data with the optical head at a variety of trial focus offsets and to subsequently examine the quality of the recorded marks. The trial focus offset which produced the highest quality recording is chosen as the optimum offset value. See, for example, U.S. Pat. No. 5,077,719 (column 5, lines 31-38). A problem with this method is that it requires both record and read back steps.
Another approach has been to calculate the best offset for reading the optical recording media or disk and to assume that it is also the best offset value for recording. See, for example, U.S. Pat. No. 4,707,648 and U.S. Pat. No. 5,142,520. This may not be a valid assumption for several reasons. For example, illumination powers for recording are significantly higher than for reading. This may affect the response of sensor elements and/or signal processing electronics. Also, there are physical changes which occur in the optical medium during recording, which of course do not occur during reading. The complicated structure of some recording media may also result in different optimum illuminating beam focus positions for reading versus recording.